Visualization Experimental Study On Evolution Behavior Of Boiling Bubble Heat Transfer In Cylinder Head Of Internal Combustion Engine

In this paper, regarding a new type of turbocharged diesel engine as the research object, boiling heat transfer and visualization experimental study in cooling water jacket of cylinder head was conducted. The arrangement and construction of the boiling heat transfer and visualization experiment platform in the cooling water jacket of the cylinder head were completed, in the experiment, shooting the evolution behavior of the bubbles in the bridge zone at different operating conditions and flow parameters, meanwhile, the basic operating parameters and the temperature of the measuring points were collected. The phenomena and mechanism of nucleation,growth, slip and coalescent of bubbles under different working conditions and flow parameters were analyzed by means of image processing, and the influence of evolution behavior of bubble on the boiling heat transfer performance was discussed.The content of the work and the main conclusions were summarized as follows:(1) Through visual images, it was found that there was boiling heat transfer phenomena in bridge zone which had larger heat flux and located in cooling water jacket of internal combustion engine under the rated condition. It was concluded from the visual images of the bubble growth that with the increase of the crank angle, the average diameter of the bubble grew gradually. In initial stage, the bubble growth rate was relatively fast. while the top of the bubble influenced by condensation, the growth rate became slow; It was analyzed from the motion characteristics of two typical bubbles in the images of sliding. The slip velocities of the bubble were 0.0514m/s and0.0852m/s respectively, which was in the accelerating slip period. While the slip velocities of the other bubble were 0.1268m/s and 0.0282m/s respectively, whose stress state had changed, the resistance increased gradually; It was found from the study on the images of bubble coalescent behavior that in different crank angle, the distribution of bubble diameter was conformed to normal distribution, and the determination coefficient of fitting curve was 98.72%. With the increase of crank angle, dispersion scale parameter σ of the bubble equivalent diameter increased.When the σ reached the maximum value 0.56, the fluctuant degree of the bubble equivalent diameter distribution was the most severe which indicated that the morebubble coalescent phenomenon, the more strong influence on randomness of the overall bubble behavior.(2) It was concluded form comparison and analysis of the growth characteristics of boiling bubbles that, under low speed heavy load condition and maximum torque condition the average growth diameter of bubble under the maximum torque condition was 19% compared with the low speed heavy load working condition.Basing the mechanism analysis, it was known that the thermal load on the heated surface under the maximum torque was more severe, and the bubble could get more heat and mass, so that it was easier to form larger size bubbles. The bubble motion characteristics under two conditions were studied, the average slip velocity of the bubble under the maximum torque condition was 0.056m/s, which was much larger than that of the low speed and high load condition 0.12m/s; It was found from the distribution rules of bubble diameter under different conditions that the average growth of the bubble’s equivalent diameter under the maximum torque condition were13% and 9.3% respectively compared with that of the low speed and heavy load condition, which showed that the influence of bubble coalescent behavior on the distribution of equivalent diameter was more obvious than that of low speed and heavy load condition.(3) The influence rules of the flow parameters such as the undercooling and the flow rate on the bubble evolution behavior were studied. It was concluded from images of bubble evolution behavior under different conditions that the average diameter of growth, average velocity of slip, and the average equivalent diameter of the bubble were gradually increased with the decrease of undercooling or flow rate. In the two conditions, the average increase of the bubble diameter were 0.12 mm and0.77 mm respectively. Single phase convective heat transfer played a leading role in the high undercooling and flow rate. The cooling fluid could effectively increase the quenching heat in the superheated fluid layer. The number of bubbles was smaller and the size of the bubble was bigger affected by the high speed fluid. Under the low undercooling or low flow rate conditions, the bubble’s stress state was stable, as a result the possibility of the slip and coalescent behavior increased, the intense disturbance between bubbles could effectively improve the performance of boiling heat transfer. Hot fluid could provide more energy and mass for the growth of bubbles,and the number of bubbles was smaller while the size was bigger.(4) Finally, it was verified by the results of boiling heat transfer of cylinder head.At the rated condition, with the decrease of flow rate, the temperature of other measuring points had the trend of slow growth, but the measured temperature of bridge zone had a slight decline, which indicated that bubble nucleation, growth, and slip and coalescent behaviors in bridge zone promoted vapor-liquid two phase mass and energy to transfer, and enhanced the heat transfer significantly. Although the flow rate maximal decreased up to 20%, the effect of single-phase convection weakened,but had no effect on the heat transfer in cooling water jacket of cylinder head,however the effect of boiling heat transfer in high heat flux area was further enhanced.